1. Field of the Invention
The present invention relates to an aperture ratio (AR) measuring apparatus for an optical recording medium having a recording surface on which pre-pits carrying information regarding the information recording tracks are repeatedly formed between such information recording tracks.
2. Description of the Related Background Art
Recently, CD-R, CD-RW, DVD-R, DVD-RW, DVD-RAM, etc. have been known as optical recording discs on which information data can be written. In addition, information recording/playing apparatuses are in practical use which can record and reproduce information data on such a recording disc.
FIG. 1 is a schematic view of the area configuration of a DVD-RW as such a recording disc.
As shown in FIG. 1, a DVD-RW has a data configuration comprising the following areas from the inner edge to the outer edge: PCA (Power Calibration Area), RMA (Recording Management Area), lead-in area, data area, and lead-out area. The PCA is an area on which data is written as a test to determine the recording power of laser beam. The RMA is an area on which management information regarding recording is written. A part of the lead-in area contains an embossed area. The embossed area contains phase pits formed on the disc beforehand. Information regarding copy prevention is written on the embossed area.
FIG. 2 shows a part of the recording surface of a disc on which data can be recorded.
As shown in FIG. 2, on a substrate 101, convex groove tracks 103 on which information pits Pt for carrying information data are formed and concave land tracks 102 are alternately formed, spirally or concentrically. Furthermore, a plurality of LPPs (land pre-pits) 104 are formed between the adjacent groove tracks 103. The LPPs 104 are formed beforehand on the land tracks 102 to indicate the recording timing and the address of information data for a disc recorder which records such information data. The groove tracks 103 are formed as grooves from the side of the substrate 101 and the LPPs 104 are formed as pits from the side of the substrate 101.
In a disc player for playing an optical disc having such LPPs, an LPP detection circuit is equipped. The LPP detection circuit includes a binarization circuit. The LPP detection circuit receives a reflection beam from the optical disc by means of an optical detector divided into two sections in the track tangential direction of a pickup, and obtains the differential signal of an output signal of the optical detector, that is, a radial push-pull signal PP. The push-pull signal PP has a waveform as shown in FIG. 3. An LPP component projects from the push-pull signal PP. A pre-pit detection signal PPD showing LPP detection is generated by comparing the level of the push-pull signal PP with a threshold value.
As shown in FIG. 4, the pre-pit detection signal PPD exhibits a pulse-shaped level change at each pickup read position corresponding to the LPP. The pre-pit detection signal PPD contains a synchronous pulse PSYNC at the beginning of each period T as shown in FIG. 4. The synchronous pulse PSYNC is followed by two pre-data pulses at a predetermined interval to indicate data such as address, etc. These pre-data pulses do not always exist at each period. As shown in FIG. 4, the third pulse from the synchronous pulse PSYNC is a pre-data pulse PD carrying a sector address. When information is recorded on an optical disc, such information is recorded by detecting the address on the optical disc based on this pre-pit detection signal PPD.
In the manufacture of an optical disc containing LPPs, the LPPs of the manufactured optical disc must conform to a prescribed LPP standard. An AR (Aperture Ratio) is measured to determine whether an optical disc conforms to said LPP standard. For AR measurement, the LPP component of the push-pull signal PP (the third LPP position from the synchronous LPP) is repeatedly sampled for a predetermined period of time. By this sampling, the overlapped waveform of the push-pull signal PP at the third LPP position, that is, an AR waveform, is indicated on a display unit such as an oscilloscope as shown in FIG. 5. It is necessary that the maximum value APmax and the minimum value APmin of the peak values from the maximum value WOmax of the groove track component of the indicated push-pull signal PP should be detected, and the AR value is calculated by the equation AR=APmin/APmax in order to confirm that such AR value is greater than a specified value. A greater AR value means that the binarization range becomes wider and pre-pit detection accuracy becomes higher.
If there is foreign matter such as dirt stuck to a disc, the foreign matter may introduce noise into the push-pull signal PP of the LPP component in AR measurement. Moreover, if such noise is generated at such a level that the minimum value APmin of the peak value of the push-pull signal PP decreases, the calculated AR value may not reach the prescribed value even though the disc conforms to the standard. Therefore, it is necessary to decrease the influence of foreign matter as much as possible in order to correctly calculate the AR value.